1. Field of the Invention
The present invention pertains to a flat panel display device, and more particularly to a method of forming a conductive pattern in an organic electroluminescent display device by using a laser ablation process and an inkjet process, an organic thin film transistor manufactured using the method, as well as a method of manufacturing the organic thin film transistor.
2. Description of the Related Art
Organic thin film transistors (OTFT) have been given much attention to as driving elements for next-generation display devices. The organic thin film transistors employ an organic layer as a semiconductor layer instead of a silicon layer. The organic thin film transistors are classified into low-molecular organic thin film transistors including oligothiophene, pentacene, or the like and high-molecular organic thin film transistors including polythiophene, depending upon materials of the organic layer.
In organic electroluminescent display devices employing such organic thin film transistors as switching elements, a plurality of pixels are arranged in a matrix on a substrate and each pixel includes two organic thin film transistors, such as a switching organic thin film transistor and a driving organic thin film transistor, a capacitor, and an organic electroluminescent element in which an organic layer is interposed between an upper electrode and a lower electrode.
Generally, a flexible organic electroluminescent display device uses a flexible substrate as a substrate, the flexible substrate generally being a plastic substrate. Since the plastic substrate has a very small degree of thermal stability, it is necessary to manufacture the flexible organic electroluminescent display device using low-temperature processes. Since the organic thin film transistors having an organic layer as a semiconductor layer can be manufactured using low-temperature processes, the organic thin film transistors have attracted much attention as switching elements for the flexible organic electroluminescent display devices.
A pentacene thin film transistor which can shorten the time for depositing a thin film and improve hole mobility is disclosed in Korean Laid Open Patent Publication No. 2004-0028010. An element structure of an organic thin film transistor which can improve electrical performance of the thin film transistor and a method of manufacturing the organic thin film transistor are disclosed in Korean Laid Open Patent Publication No. 2004-0084427. A thin film transistor in which a channel region is made of organic compound having radicals and which can improve the carrier mobility and the ratio of on current to off current is disclosed in Japanese Laid Open Patent Publication 2003-92410.
An organic thin film transistor includes a gate electrode formed on a substrate, a gate insulating layer formed on the substrate and on the gate electrode, source and drain electrodes formed on the gate insulating layer, and a semiconductor layer formed on the source and drain electrodes and on the gate insulating layer.
In the organic thin film transistor, since conductive patterns such as the gate electrode and the source and drain electrodes are formed using a photolithography process, there is a disadvantage in that the processes are complex and the processes must be performed at a low temperature due to the small degree of thermal stability of the substrate. On the other hand, an inkjet method is known as the method of forming a conductive pattern such as the source and drain electrodes and the gate electrode. A method of forming source and drain electrodes by applying an electroless gold plating solution on a substrate in an inkjet method, heating the solution at 90° C. to form a gold thin film pattern is disclosed in Korean Laid Open Patent Publication No. 2004-0029402. As described above, in the method of forming source and drain electrodes by using an inkjet method, the source and drain electrodes are formed by pattern-applying and then curing a solution including a material for the source and drain electrodes on the substrate.
Since the method of forming source and drain electrodes employs the inkjet method, there is an advantage in that the processes can be simplified and performed at a low temperature due to omission of a photolithography process. However, there is a problem in that pinning points are generated with evaporation of a solvent at the time of applying and then curing the material for the source and drain electrodes and that a coffee stain effect that occurs at edges of a pattern and becomes relatively thick results.
Such an effect increases as the evaporation speed of the solvent increases. In addition, when line patterns such as the source and drain electrodes and the gate electrode are printed as in the related art, the effect becomes more severe, thus forming a sectional profile having protrusions at the edges.
A thin film transistor having a bottom gate structure has a sectional structure that a gate insulating layer is formed on a gate electrode. Therefore, when the gate electrode is formed using the inkjet method, the thin film transistor has the sectional profile that damages the gate insulating layer formed thereon. Therefore, what is needed is a method of making a device and a device where low temperature processes only are used on a flexible substrate that overcomes the above problems.